1. Field of the Invention
The present invention relates to a pattern forming method utilizing a positive resist composition usable in a lithography process for manufacturing semiconductors, such as ICs, and circuit boards for LCDs and thermal heads, and other photofabrication processes. More specifically, the invention is concerned with a pattern forming method utilizing a positive resist composition suitable for exposure performed with a projection exposure apparatus for immersion lithography using as a light source far ultraviolet rays with wavelengths of 300 nm or below.
2. Description of the Related Art
With the growing need for finer semiconductor devices, it has been advanced to adopt exposure light sources having shorter wavelengths and projection lenses having higher numerical apertures (higher NAs). Up to now, steppers using as light sources ArF excimer laser with a wavelength of 193 nm and having NA of 0.84 have been developed. As generally well known, the resolution and the focal depth of these machines can be given by the following expressions;(Resolution)=k1·(λ/NA)(Focal depth)=±k2·/NA2 where λ is the wavelength of a exposure light source, NA is the numerical aperture of a projection lens, and k1 and k2 are coefficients concerning a process.
Although steppers using as light sources F2 excimer laser with a wavelength of 157 nm are under study with an eye toward achieving higher resolution by further shortening of the wavelengths of exposure light sources, it is very difficult to stabilize production costs and qualities of apparatus and materials since lens materials used in exposure apparatus and materials used for resist in order to ensure shorter wavelengths are confined within very narrow limits, and fears are arising for completion of exposure apparatus and resist having sufficient performance and stability within a required period.
As an art of heightening the resolution in an optical microscope, the method of filling the space between a projection lens and a test specimen with a liquid having a high refractive index (hereinafter referred to as an immersion liquid), or the so-called immersion method, has hitherto been known.
This “immersion effect” can be explained as follows. In immersion lithography, the foregoing resolution and focal depth can be given by the following expressions;(Resolution)=k1·(λ0/n)/NA0 (Focal depth)=±k2·(λ0/n)/NA02 where λ0 is the wavelength of a exposure light source in the air, n is the refractive index of an immersion liquid relative to the air and NA0 is equal to sin θ when the convergent half angle of incident rays is represented by θ. That is to say, the effect of immersion is equivalent to the use of exposure light with a 1/n wavelength. In other words, application of the immersion method to a projection optical system having the same NA value can multiply the focal depth by a factor of n.
This art is effective on all shapes of patterns, and besides, it can be used in combination with super-resolution techniques under study at present, such as a phase-shift method and an off-axis illumination method.
Examples of apparatus utilizing this effect for transfer of fine circuit patterns in semiconductor devices are disclosed in JP-A-57-153433 and JP-A-7-220990, but these documents have no description of resist suitable for immersion lithography.
Recent progress of immersion lithography is reported in Proceedings of International Society for Optical Engineering (Proc. SPIE), vol. 4688, p. 11 (2002), J. Vac. Sci. Technol. B, 17 (1999), Proceedings of International Society for Optical Engineering (Proc. SPIE), vol. 3999, p. 2 (2000), and WO 2004/077158.
In order to supplement the sensitivity drop by light absorption from the resist for KrF excimer laser (248 nm) onward, the image forming method referred to as a chemical amplification method has been adopted as a method of patterning resist. To illustrate an image forming method utilizing chemical amplification by a positive-working case, images are formed in a process that exposure is performed to cause decomposition of an acid generator in the exposed areas, thereby generating an acid, and conversion of alkali-insoluble groups (groups insoluble in an alkaline developer) into an alkali-soluble groups (groups soluble in an alkaline developer) by utilizing the acid generated as a reaction catalyst is caused by bake after exposure (PEB: Post Exposure Bake) to enable the exposed areas to be removed by an alkaline developer.
In a general resist process used for formation of resist patterns, a resist composition is coated evenly on a substrate by use of a spin coat method, the substrate is heated in order to evaporate a resist solvent, the substrate is cooled to room temperature, the resist coating is exposed to light via a mask bearing the desired patterns, PEB is carried out immediately after the exposure, the resist coating thus treated is immersed in a developer after cooling to room temperature, the developer is rinsed out with water, and then the thus patterned resist coating is dried by a spin dry method.
Application of immersion lithography to chemical amplification resist according to the foregoing general resist process requires further improvements in development defects appearing after development.